Recombinant protein expression systems using insects are emphasized because they can produce modified proteins similar to those produced in mammalian cells in relatively large amounts. In systems using insects, it has been investigated to use nucleopolyhedrovirus (NPV), in particular, BmNPV or AcNPV as a vector, in which BmNPV can infect Bombyx mori, and AcNPV can infect Autographa californica (one of Plusiinae) and Spodoptera litura, but usually does not infect silkworm. From the viewpoint of host cells, the systems using insects are roughly classified into systems using culture cells and systems using insect individuals. In recent years, AcNPV-Spodoptera litura culture cell lines have been widely used, while BmNPV-silkworm individual systems are now in the process being used.
Systems using insect culture cell lines have advantages over systems using insect individuals, that is, they can treat a large number of specimens without human intensive.
Silkworm-derived culture cells hitherto reported are, for example, those derived from embryonic tissue slices of silkworm strain “Kuroko” (Non-Patent documents 1 and 2), those derived from embryonic tissue of silkworm strain “J125” (Non-Patent documents 3 and 4), those derived from embryonic tissue of silkworm strain “C129” (Non-Patent document 3), those derived from an embryonic head section of silkworm strain “Gominhaku” (Non-Patent document 5), and those derived from embryonic tissue slices of silkworm strains “Murasakiko”, “Black moth”, “Daizo”, and “Sepia-lumazine” (Non-Patent document 6).    Non-Patent document 1: Inoue, H. and Mitsuhashi, J., A Bombyx mori cell line susceptible to a nuclear polyhedrosis virus; J. Seric. Sci. Jpn. 53, 108-113 (1984).    Non-Patent document 2: Inoue, H., Taniai, M., and Kobayashi, J., Kaiko no fuchaku-sei baiyosaibo-kei no juritsu to sono tokusei (Establishment and characterization of adhesive cell lines of silkworm); Sanshi-Konchu Ken-Ho (Bull. Natl. Inst. Seric. Entomol. Sci.) 1, 13-25 (1990).    Non-Patent document 3: J. Seric, Sci. Jpn. 57(3), 184-188 (1988).    Non-Patent document 4: JARQ 26, 196-202 (1992).    Non-Patent document 5: Imanishi, S., Kaiko taieki ni hannou shie kaitai henka wo okosu baiyousaibo-kei no sakusyutsu (Establishment of culture cell line producing morphology change by silkworm body fluid); Nichi-San Kanto Shibu Koyo (Abstracts Jpn. Seric. Kanto Dev.) 50, 40 (1999).    Non-Patent document 6: Imanishi, S., Cho, E. S., and Tomita, S., Novel Bombyx mori cell lines cultivable at 37° C.; Appl. Entomol. Zool. 34, 259-266 (1999).